[clang] [clang-format]: Add `Custom` to `ShortFunctionStyle`; add new AllowShortFunctionsOnASingleLineOptions for granular setup (PR #134337)

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sstwcw wrote:

When I changed the `AlignConsecutiveAssignments` option, I was told to reuse the same option name.  Now the program allows the user to specify the option as either a string or an associative array.

https://github.com/llvm/llvm-project/pull/134337